In a downlink of a mobile communications system, a radio base station may share one physical channel between the radio base station and mobile stations belonging to the radio base station. The physical channel used in this case is referred to as a downlink shared channel hereinafter.
In the downlink shared channel, the radio base station controls the order of packet transmissions to plural mobile stations as communications parties in accordance with an instantaneous transmission (radio) quality of each mobile station, thereby enhancing throughput, or so-called system capacity, of the radio base station.
Such transmission order control by the radio base station is call scheduling. It is known that application of such a scheduling to packet data transmission may increase a communications capacity and enhance a communications quality (see non-patent document 1). The scheduling is based generally on the premise that requirements for packet data handled by the scheduling are not very strict in terms of transmission latency.
By the way, regarding standardization of the third generation mobile system, also known as IMT-2000, 3GPP (Third Generation Partnership Project) and 3GPP2 (Third Generation Partnership Project 2) organized by provincial standardization facilities have formulated standard specifications related to W-CDMA and cdma 2000, respectively.
3GPP has standardized “High Speed Downlink Packet Access (HSDPA)”, which is a high speed downlink packet transmission method, in anticipation of an increase in high speed and high volume traffic by downloading or the like from data bases or Web sites through downlink (see non-patent document 2, for example).
In addition, 3GPP2 has standardized “1x-EVDO”, which is a downlink high speed data only transmission method, from the same point of view (see non-patent document 3) Here, “DO” is an abbreviation of “data only” in the cdma 2000 1xEV-DO.
HSDPA combines control of radio channel modulation methods and coding rates in accordance with transmission (radio) quality between a mobile station and a radio base station, such as Adaptive Modulation and Coding Scheme (AMCS), and scheduling performed in units of a few milliseconds, thereby enhancing throughput for each user and throughput for a system as a whole.
When it comes to a scheduling algorithm that controls the transmission order of packets waiting to be transmitted in a radio base station, round robin scheduling has been well known, in which the downlink shared channel is allocated to mobile stations belonging to the radio base station by turns (for example, in an order of mobile stations #1->#2->#3).
In addition, there have been known MAX C/I (Maximum C/I) and Proportional Fairness scheduling in which the transmission order of the packets waiting to be transmitted is controlled in accordance with a transmission quality and an average transmission quality of each mobile station.
An example of the control method based on the general Proportional Fairness scheduling will be described.
Proportional Fairness is a scheduling algorithm that can perform allocations responsive to an instantaneous change in downlink transmission quality of each mobile station while assuring fairness between mobile stations.
Referring to FIG. 1, a Proportional Fairness scheduler is described. FIG. 1 is a flowchart indicating operations of the Proportional Fairness scheduler. In this scheduling algorithm, an average transmission quality and a transmission quality of each mobile station are measured as elements of an evaluation function; the evaluation function is obtained for each mobile station belonging to the radio base station; and to the mobile station having the largest evaluation function is allocated the downlink shared channel. By the way, an average transmission rate is used as an example of the average transmission quality in the following.
In FIG. 1, initial values are determined at step S41 as follows.
[Initial Values]
n=1 (n: mobile station suffix)
Cmax=0 (Cmax: the maximum value of the evaluation function)
nmax=0 (nmax: the mobile station having the maximum value of the evaluation function)
Then, elements necessary to calculate the evaluation function, specifically, (1) the transmission quality of each mobile station, (2) an average transmission rate, are measured at step S42. Next, the evaluation function Cn is calculated using the measured values obtained at step S42.
      C    n    =            R      n                      R        _            n      
Rn: instantaneous radio quality of mobile station n
 Rn: average transmission rate of mobile station n
Then, it is determined at step S44 whether the calculated evaluation function Cn obtained at step S43 exceeds the Cmax. Since Cmax is zero in this case, the determination made at step S44 is YES; the value of Cn calculated at step S43 is set to Cmax; and nmax is set to 1.
Next, n is incremented at step S46 and the values of the evaluation function are obtained by turn for the mobile stations communicating with the radio base station by a loop process at step S47. Then, the mobile station having the largest evaluation function is selected and to this mobile station is allocated the downlink shared channel.
Since the Proportional Fairness scheduler allocates the downlink shared channel to a mobile station having relatively better transmission quality, a higher throughput is expected compared with the round-robin scheduling.
In addition, since the instantaneous transmission quality is divided by the average transmission rate in each mobile station, the evaluation function value is reduced in the mobile stations having a higher average transmission rate, thereby realizing allocations with high temporal fairness.
Moreover, in packet transmission in a packet network, it is now under consideration that two priority classes are to be provided and packets having a first priority are to be preferentially transmitted compared with packets having a second priority (see patent-related document 1, for example).
However, in the Proportional Fairness scheduler, it is difficult to control packet allocation depending on whether a mobile station concerned has retransmission data.
Since it is difficult to control the packet allocation depending on whether a mobile station concerned has retransmission data, there may be a case where packets are not assigned to the retransmission data. When the retransmission data remain in a lower layer, there is caused a delay in upper layers, such as an RLC layer and a TCP layer, or the retransmission data may be discarded after a predetermined time period measured by a discard timer. Therefore, there is a disadvantage in that an error rate becomes higher in the upper layers as a result.
Furthermore, when packet transmissions are performed in accordance with the priority class, packet allocation is required in which the priority class and presence/absence of the retransmission data are both taken account of. However, it is difficult in the case of the Proportional Fairness scheduler to take account of the priority class and the presence/absence of retransmission data when controlling the packet allocation.
By the way, data are considered to be retransmission data when the data have been mapped at least once in the HS-DSCH and transmitted to a mobile station concerned, when Ack for the data is not received from the mobile station through the uplink HSDPA control channel HS-DPCCH, and when elapsed time from the previous transmission becomes a predetermined time or more.    Non-patent Publication 1: J. M. Holtzman, IEEE VTC2000, Spring    Non-patent Publication 2: 3GPP TR25.848 v. 4.0.0    Non-patent Publication 2: 3GPP C.S0024 Rev. 1.0.0    Patent-related Publication 1: Japanese Patent Application Laid-Open Publication No. H03-58646.